First off when a scientist harvests the embryo, it is only a small cluster of cells. Though these cells have the potential to grow into more at this point it does not even have a heartbeat. Embryonic stem cells have the potential to grow into many sorts of tissue, and thus to regenerate damaged or dead organs or cells. Stem cells from adults and from umbilical cords and placentas also show therapeutic promise, but many researchers believe that embryonic cells are most versatile and thus offer the best hope of reversing Parkinson's and Alzheimer's disease by regenerating brain tissue, cystic fibrosis and spinal injuries by